Lecture Notes on Mendel's Experiments

Introduction to Mendel's Work

• Focus on Gregor Mendel's experiments and contributions to the study of heredity.
• Mendel chose peas for his experiments due to their variety and ease of breeding.
  • Varieties included: tall, short, green, yellow, round, wrinkled.
  • Peas grew well in gardens and could be used as food.

Methodology of Mendel's Experiments

Initial Control Experiments

• Step 1: Grew different pea varieties separately to confirm true breeding.
  • Round peas bred true to round.
  • Wrinkled peas bred true to wrinkled.
• Importance of control experiments for reliable data.

Crossing Experiments

• Step 2: Crossed round peas with wrinkled peas (F0 generation).
  • Result in the F1 generation: all offspring were round.
  • This contradicted the blending inheritance theory, as no intermediate traits appeared.

Self-Pollination

• Step 3: Selfed the F1 round peas.
  • Result: offspring included both round and wrinkled peas.
  • Reinforcement of the discrete nature of traits.
  • Observation that traits can reappear after disappearing.

Quantitative Analysis

• Mendel counted the offspring's traits.
  • Found a ratio of approximately 2.96:1 for round to wrinkled peas (not the expected 3:1).
  • Hypothesis: the ratio was attempting to approximate 3:1.
• Developed a model of inheritance based on dominant and recessive alleles.
  • Alleles: variants of a gene (e.g., big R for round, little r for wrinkled).
• Mendel's model explained inheritance patterns using a Punnett square concept (not originally used by Mendel).

Peer Review Process

• Mendel submitted his findings for publication in the Proceedings of the Royal Society.
  • Faced peer review, where scientists evaluated his work and requested further predictions.
• Key predictions included:
  • Selfing round peas would yield a ratio of round to wrinkled.
  • Crosses between different peas would yield expected ratios based on the model.

Scientific Process

• Importance of making predictions and testing them to validate hypotheses.
• Highlighted the iterative nature of scientific investigation.

Key Definitions in Genetics

• Gene: A discrete factor of inheritance.
• Allele: Alternative forms of a gene (e.g., big R and little r).
• Genotype: The genetic makeup of an individual (e.g., big R, little r).
• Phenotype: The observable traits of an individual (e.g., round or wrinkled peas).
• Dominant and Recessive:
  • Phenotype 1 is dominant if the F1 heterozygote exhibits phenotype 1.
  • Importance of distinguishing between dominant/recessive phenotypes and alleles.
• Homozygote: Individual with identical alleles.
• Heterozygote: Individual with different alleles.

Conclusion

• Mendel's work laid the foundation for modern genetics, emphasizing the importance of controlled experiments, quantitative analysis, and clear definitions in scientific discourse.